Chylous effusions
Chylous effusions or chylothorax occurs when there is lymphatic chyle in the chest cavity and/or pleural space as a result of intrinsic abnormalities of the lymphatic system or secondary to thoracic duct injury (trauma, surgery, malignancy, cardiovascular disease). Primary disorders of pulmonary and pleural lymphatic flow predispose to impaired lymphatic drainage, increased trans-pleural filtration pressure (lymphatic hypertension), retrograde lymphatic flow and increased permeability that results in chylous effusion or chylothorax(Figure 1b) . These disorders are sometimes referred to as pulmonary Lymphatic Perfusion Syndromes (PLPS). Chylous effusions typically present with respiratory symptoms such as dyspnea, chest pain, cough, and fatigue.
Abnormal development of the native lymphatic system presents as lymphangiomatosis or lymphangiectasia. Lymphangiomatosis is a generalized term for the over-proliferation of malformed and poorly functional lymphatics in the lungs (Figure 1b) . Abnormal lymphatics can infiltrate a single organ or present as multifocal disease with infiltrations into bone and surrounding tissue. Pulmonary lymphangiomatosis can present with non-specific respiratory symptoms of progressive cough, wheezing, or dyspnea. Congenital pulmonary lymphangiectasia is characterized by dilated lymphatics in the subpleural and interlobular lymphatic vessels that fail to drain appropriately. Primary pulmonary lymphangiectasia is hypothesized as a failure of the normal large lymphatic channels to regress mid-lung development (at ~20 weeks). Lymphangiectasia commonly presents with non-specific respiratory distress.
Chyle aspirated via thoracentesis is milky in appearance with lymphocyte predominance and biochemical analysis often with a triglyceride concentration >110 mg/dL. In the unfed state, chylous fluid can be clear yellow in the absence of chylomicrons.